This invention relates generally to the control of pollutants from combustion processes. More particularly, this invention relates to a technique for enhancing NOX reducing catalyst activity and thereby efficiently removing NOX from a combustion process gas stream.
The 1990 Clean Air Act Amendments require major sources of air emissions to limit the discharge of NOX . NOX is present in the flue gas emitted from combustion processes. Therefore, cost-effective methods for controlling NOX are of significant interest.
Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR) catalysts for NOX removal are known in the art. Conventional NOX SCR catalysts require large amounts of catalyst and the flue gas stream to be at relatively high temperatures (between approximately 300 to 400xc2x0 C.) in order to have sufficient activity for effective NOX reduction. In such schemes, ammonia or urea are also typically added as a reducing agents. However, significant problems are associated with the use of reducing agents, including, for example the formation of ammonium compounds from ammonia, referred to as ammonia xe2x80x9cslip.xe2x80x9d Ammonia xe2x80x9cslipxe2x80x9d occurs when unreacted ammonia and ammonium compounds pass out of the SCR unit. Such ammonia xe2x80x9cslipxe2x80x9d can plug downstream air heaters and impact ash use and disposal. SNCR is not as effective as SCR processes and the use of urea in SNCR also produces unwanted ammonia and ammonium compounds. Therefore, it would be desirable to have a process than can supplement NOX reduction downstream of the SCR/SNCR process, where the process can also reduce ammonia slip.
In addition to SCR and SNCR, low NOX burners (LNB) positioned in the furnace are also commonly used for NOX reduction. However, the LNB""s are not as effective as SCR/SNCR at removing NOX from the combustion process. Therefore, it would also be desirable to have a process that can supplement NOX reduction downstream of LNB burners to meet more stringent NOX emissions control requirements.
There are ongoing efforts to develop low temperature catalysts for applications between approximately 100 to 250xc2x0 C. Unfortunately, these low temperature catalysts are sensitive to high SOx concentrations in the flue gas. There are also NOX SCR catalysts being developed that are reagentless, so, for instance, ammonia is not required as a reducing agent. These catalyst systems can benefit from the imposition of additional and alternative activating agents to the catalyst so that they can operate at lower temperatures, be less sensitive to poisoning agents, and more reactive without the addition of chemical agents.
In view of the foregoing, it would be highly desirable to provide an approach to enhance NOX removal. Ideally, the technique would reduce the amount of catalyst needed, or would operate at relatively low temperatures, or reduce the amount of required chemical reducing agents.
According to the invention there is provided an apparatus for decreasing the concentration of contaminants, such as NOX or reducing agents, present in a gas stream. The apparatus comprises a NOX reducing unit, a duct to receive a flue gas stream from said NOX and reducing unit, and an activation source associated with the duct. In use, the activation source applies energy to the flue gas stream to facilitate the removal of contaminants from the flue gas stream.
Further, according to the invention there is provided a method of decreasing the concentration of contaminants within a flue gas stream. An activation source is provided downstream of a NOX reducing unit. The activation source is associated with a duct configured to convey a flue gas stream. The activation source is then activated to facilitate the removal of contaminants from the flue gas stream.
In this way, the inclusion of an activation source downstream of a NOX reducing unit decreases the concentration of NOX and reducing agents within the flue gas stream. The activation source of the invention may also be used with a low NOX burner to reduce NOX even in the absence or reducing agents that are present in other embodiments of the invention.